Oh, you could use 'ferret':
Compared to [the lumbering, clumsy] Internet Explorer, Midori is a ferret of a browser.
Ferrets being small, fast animals that punch considerably above their weight. Naturally, another figurative sense of 'ferret' is helpful in this context:
To uncover and bring to light by searching.
[ferret. (n.d.) American Heritage® Dictionary of the English Language, Fifth Edition. (2011). Retrieved November 5 2016 from http://www.thefreedictionary.com/ferret.]
Not knowing the audience, and sticking to the animal kingdom, I'm avoiding anything the least bit literary, such as
Compared to [the elephantine] Internet Explorer, Midori is a serpentine browser.
'Elephantine' means 'like an elephant', and 'serpentine' means 'like a serpent'.
Similarly, I'm avoiding the touch of cliche involved with bringing the realm of natural phenomena to bear:
Compared to [the glacial] Internet Explorer, Midori is a lightning strike browser.